1. Field of the Invention
The present invention relates to a method of producing an ion sensitive film used in an ion sensor (especially, an ion selective film) that is employed to measure the activity of ions within a liquid solution.
2. Description of Related Art
In recent years, ion sensors (especially, ion selective electrodes) for medical applications have been vigorously used to quantify ions (e.g., sodium, potassium, and chlorine ions) dissolved in a liquid within a living organism, such as blood or urine.
An existing ion sensor is shown in FIG. 1 and comprises a sensor (electrode) body 1 and an ion sensitive film 4 fixed to the front-end surface of the body 1. The inside of the body 1 is filled with an internal electrolyte 3. An internal electrode 2 made of silver chloride is immersed in this internal electrolyte.
Referring next to FIG. 2, this ion sensor, indicated by numeral 5, is immersed in a sample solution 6 together with a salt bridge 7. On the other hand, a reference electrode 9 is immersed in an aqueous solution 10 of saturated potassium chloride together with the salt bridge. The potential difference between both electrodes is read by an electrometer 8. Using a reference sample having a known concentration as an object to be measured, the electromotive force is measured, and a calibration curve is created. The ion concentration of the sample under investigation is found by comparing the electromotive force produced by the sample against the calibration curve.
The performance of such an ion sensor also depends on the performance of the used ion sensitive film. Known ion sensitive films used for ion sensors include (a) a film produced by preparing a film support material made of polyvinylchloride and mixing an ion sensitive material into the film support material together with a plasticizer, (b) an ion-exchange film, and (c) a “film-like substance” (disclosed in Japanese Patent No. 2504513) consisting of a polymer having a quaternary ammonium group having a certain structure.
However, the ion sensor using the ion sensitive film of the type (a) above has the disadvantage that the electrode life is short, because the ion sensitive material within the film gradually dissolves into the liquid solution.
The ion sensor using the ion sensitive film of the type (b) has a short life because ionic groups are introduced into a polymer by covalent bonding, the polymer forming a film. It has been pointed out, however, that the film is an ion-exchange film generally used for electrolytic purposes and so if the film is used as an ion sensitive film in an ion sensor, the film is affected greatly by interfering ions.
In the ion sensitive film of the type (c) as disclosed in the above-cited Japanese Patent No. 2504513, a straight-chain polymer is formed by polymerizing monomers having a quaternary ammonium group of a certain structure. Thus, the film shows excellent response to chlorine ions and good waterproofness. The ion selective electrode has a long life and can measure chlorine ions with high sensitivity stably.
The ion sensitive film of the type (c) is formed by normal heating radical solution polymerization using a radical polymerization starter and, therefore, limitations are placed on the degree of polymerization. In addition, molecular weights are broadly distributed. Therefore, it can be said that there remains room for improvements of durability and life. In addition, in such normal radical solution polymerization, it takes a long polymerization reaction time to increase the reaction rate. Furthermore, unreacted monomers remain. Additionally, it is laborious to refine the reaction mixtures.